This invention relates generally to an improved power control arrangement for an automatic surface unit in a cooking appliance such as a domestic electric range. More specifically, this invention is an improvement to the power control arrangement disclosed and claimed in commonly assigned U.S. Pat. No. 4,493,980 to Thomas R. Payne, the specification of which is hereby incorporated by reference.
The electronic control arrangement of the above referenced patent provides a significant improvement in the temperature control performance of automatic surface units over the conventional electromechanical sensing and control devices conventionally used for such surface units. In that control arrangement the applied power level for the surface unit is adjusted as a function of an error signal which is directly proportional to the difference between the selected utensil temperatures range and the sensed utensil temperature range. This error signal is large early in the transient heat up phase when the differential is large resulting in a relatively high applied power level and goes to zero as the differential goes to zero with the applied power level diminishing accordingly. Consequently, the unit is greatly overdriven initially to heat up the utensil rapidly, but only slightly so as the sensed temperature approaches the selected steady state temperature range to minimize overshoot. This arrangement works well for relatively small and average thermal loads. However, for relatively large thermal loads, during steady state operation, that is operation after having initially reached the selected steady state temperature range, the utensil temperature may drop below the desired minimum steady state temperature, a condition referred to as undershoot. When such conditions occur the error signal at least initially is relatively small, and consequently the applied power level for the surface unit is such that utensil temperature may be undesirably slow in returning to the selected range.
Therefore, it is an object of the present invention to provide an improved power control arrangement which will retain the rapid thermal response with minimum overshoot during the transient heat up phase provided by the arrangement of the '980 patent, yet which will provide a more rapid thermal response to undershoot conditions occurring during operation in the steady state phase.